The development of soft ionization techniques such as matrix assisted laser desorption ionization (MALDI) has greatly changed the field of mass spectrometric analysis. MALDI-MS benefits from its salt tolerance, simplicity of mass spectra, and broad mass range. Because of the interference from matrix-related ions in low mass range, MALDI-MS is seldom applied to the analysis of low-molecular weight compounds (below 600 Da). This limitation has hampered its wide application in important research fields such as drug discovery and biotechnology, where small molecule detection and identification is of utmost significance. In recent years, various methods leading to direct desorption/ionization without organic matrices have been extensively explored, and surface-assisted laser desorption/ionization (SALDI) has gained considerable attention and has found a broad range of applications in environmental, genomics and proteomics fields owing to its attractive features of simple sample preparation and low background ions. SALDI relies on direct absorption of UV laser light by the substrate or its coating that lead to molecular desorption and subsequent ionization. A range of materials have been investigated for their effective use in SALDI. Nanomaterials in different forms including inorganic powder, nanowire, nanotubes and porous thin structures have been tested as alternatives to organic matrices. Metallic nanoparticles are another family of materials that have been heavily explored. These materials show promising results but also exhibit limitations and have suffered from problems such as inhomogeneous deposition, molecular degradation and interference from metal cluster ions. Among all substrates studied so far, porous silicon (also referred to as desorption/ionization on silicon, DIOS) is the most significant and well-utilized as it offers highly effective ionization due to strong UV adsorption and heat transfer. DIOS substrates are typically prepared by an electrochemical etching procedure. Yet DIOS-MS has limited upper mass range, and the surface is susceptible to oxidation deactivation and requires stringent control of surface physical properties, which must be realized through careful selection of the silicon type and etching conditions, whereas nonuniform surface structures can substantially deteriorate the performance.
A new class of materials based on laser induced electron-phonon interactions for effective desorption/ionization and thus matrix-free mass spectrometric analysis of a range of biomolecules is described herein. In accordance with an exemplary embodiment, a nanoscale, glass-like silicate film fabricated on a thin gold substrate through a layer-by-layer (LbL) deposition/calcination process (FIG. 1) is utilized. There have been reports in literature that use silica (SiO2) and silicate-based materials for SALDI-MS analysis of small molecules. These methods utilize either a sprayed coating of a sol gel solution or a homogenized particle suspension to load the silica to the sample stage for enrichment of samples in SALDI analysis. As a result, the thickness of these layers is in the range of 500-1000 μm and the uniformity is difficult to control. The LbL/calcination process as developed at UC Riverside and described herein, on the other hand, generates a vastly different substrate and allows for precise control of the coating thickness and porosity in the nanometer scale, which is crucial to this ionization method.
In accordance with an exemplary embodiment, the effectiveness of calcinated films on gold for SALDI-MS detection of amino acids, peptides and proteins has been systematically investigated and the substrates are characterized by a number of techniques including scanning electron microscopy (SEM), atomic force microscopy (AFM) and contact angle measurement to understand the relationship between surface property and performance. The ultrathin glassified coating is stable and has high durability, and has a number of other advantages including low cost, well-defined surface property, reusability and ease of preparation and functionalization. In addition, the photonic properties of the thin gold substrate can allow for multiple modes of detection at the same surface and development of new hyphenated technologies.